Knock Knock. Who's there? It's your engine, informing there's a problem somewhere under the hood. If the smooth rumble which previously came from your engine is replaced with a repetitive tapping or pinging sound that becomes louder and faster as you accelerate, that's a classic sign of engine knock.
But "engine knock" can describe a variety of different noises that can each be caused by different individual problems, which makes it tricky to determine the source of the engine. To help you get started, we've listed six of the most common causes of engine knock below.
But before that, let us explain the term “Detonation Knock”. When your engine is running smoothly, as it should, the air/fuel mixture burns up in a single, controlled detonation inside each cylinder. Detonation knock is a knocking noise that you'll hear when the air fuel mixture in the cylinders is detonating in more than once place at a time. Parts of the air/fuel mixture can start to ignite prematurely. When these mini fireballs collide, they create a knocking noise.
Most common causes:
Detonation Knock: Incompatible Spark Plugs
If any spark plugs other than what is recommended by the manufacturer is used, it can cause the knocking sound you hear. The spark plug has a certain heat range, which means it withdraws heat from the combustion chamber. Using the wrong part can prevent it from working correctly. It's also common for engine knocking to occur when the spark plug gap is not correctly set.
The spark plug gap is the place where the spark plug ignites the air-fuel mixture that helps power the car. Too narrow of a gap creates a spark too weak to perform this task, and too wide a gap might stop a spark from firing at all, or to only misfire rapidly.
Detonation Knock: Carbon Deposits
Fuels for vehicles are required to have a carbon cleaning detergent, but they may not prevent carbon deposits from forming. As fuel mixes with oxygen, it is burned. Since petrol and diesel are made up of multiple carbon molecules, the residual of carbon will form on valves, spark plugs, and other components involved in the combustion process. This reduces the amount of volume inside the cylinder and increases the amount of compression.
In most cases, using a special fuel injector cleaner or additive designed to clean carbon deposits on engine components can resolve this issue.
Less common causes
Detonation Knock: Too Low Octane
If your car has a performance-tuned engine and the manufacturer specifies your vehicle requires high-octane fuels (Subaru STI’s for example) , you could experience engine knock if you put in fuel with too low of an octane rating. High octane fuels resist knock. Be sure to check your owner's manual to find out what octane rating is right for your car.
Detonation Knock: Bad Timing
On most modern cars, the timing of the engine -- which is the point in the engine's travel the spark plugs will fire -- is controlled by the computer. But, if for any reason the spark isn't firing exactly when it should, this can cause multiple detonations in the cylinder, leading to engine knock.
Detonation Knock: Lean Air/Fuel Mixture
Problems with the oxygen sensors, fuel injectors, fuel pump or mass airflow sensor can create a lean air/fuel mixture in the engine. A lean air/fuel mixture is one that doesn't have enough fuel and too much air. Without enough fuel in each cylinder, the mixture won't burn fast enough, allowing for multiple detonations -- that's engine knock.
Detonation Knock: Bad Knock Sensor
Luckily, engine knock isn't a frequent problem with modern cars, because the air/fuel ratio, fuel injectors and timing are all computer controlled. There's even a knock sensor that's designed to detect engine knock and tell the Engine Control Unit, so it can correct the problem automatically.This naturally means that a bad knock sensor can allow the engine to knock. If you're driving around in a modern car with engine knock, checking the knock sensor is an important part of the diagnostic process.
Rod Knock: Worn Bearings
Another type of engine knock is rod knock. As the pistons travel up and down in the engine, they turn the crankshaft which ultimately sends power to the wheels. The rod bearings facilitate smooth piston movement, but they can become worn out or come out of position over time.As the bearings wear out, the pistons will start to rattle against the crankshaft, creating a very similar knocking sound. To fix this problem, you may need new bearings or other work done on the pistons or crankshaft -- parts located deep in the engine, meaning this can be a time-intensive repair.
Other sources of sounds similar to “knocking”
Ticking Noise In Cylinder Head: Insufficient Cylinder Lubrication
It is imperative for the upper area of the cylinder head to receive oil for lubrication purposes. If the cylinder head is not adequately lubricated in this area, then you can expect knocking noises to form.This is usually the result of loose lifters or valves which may not be getting the proper amount of oil they need. In any event, check to ensure that your car always has oil enough oil, and that it is flowing properly to the cylinder head area. Either you have an oil leak or perhaps your oil is too old and needs to be replaced.
Bad Belt Tensioners / Pulleys
Another possible source of engine knocking isn't actually coming from the engine itself at all. It could be coming from the accessory belt. As the engine runs, it turns a belt that's connected to various pulleys throughout the engine bay. This belt must be under the exact right amount of tension so that it turns smoothly and quietly. If the belt becomes stretched out, the tensioner isn't working properly, or one of the pulleys becomes bent out of shape, you can hear rattling, clicking and slapping noises that might be mistaken for engine knock
Luckily, fixing a noisy accessory belt can be as simple as replacing the belt, adjusting the tension or replacing a bad tensioner or accessory pulley.
Resolving engine knocking problems is not always simple or something most car owners can do themselves. At the first sound of knocking, it would be prudent to contact a professional technician to correctly diagnose the source of the knocking and recommend the right repairs.
Once you know what is wrong, and what parts need to be replaced – be sure to have a look on our website or reach out to us. We pride ourselves in being consistently able to provide you with the right part at the right price, and get you back on the road as promptly as possible!